Coolant control for hot strip mill

ABSTRACT

The disclosure of the present invention relates to a strip temperature control method and apparatus for a finishing train of a hot strip rolling mill, including strip interstand coolant supply means, and a control for varying the rate of flow thereof, which includes a feed-forward control signal wherein the rate of flow is controlled in certain stands with reference to a computed temperature and a desired temperature of the strip as it leaves the train, and a feed-backward control signal wherein the rate of flow is controlled in one or more stands with reference to the actual temperature and a desired temperature as it leaves the train.

United States Patent [1 1 Greenberger 1 COOLANT CONTROL FOR HOT STRIP MILL [75] Inventor: Joseph Irwin Greenberger,

Pittsburgh, Pa. [73] Assignee: Wean United, Inc., Pittsburgh, Pa. [22] Filed: Mar. 2, 1972 [21] Appl. No.: 231,221

[52] 11.5. CI 72/13, 72/201, 72/364, 134/57, 266/6 S [51] Int. Cl. B2lb 1/22, B21b 45/02 [58] Field of Search 72/8, 9, 13, 200, 72/201, 202, 342, 364; 184/15, 64, 57; 266/4 S, 6 S, 3 R

[56] References Cited UNITED STATES PATENTS 3,300,198 1/1967 Clumpner et a1 .1 266/6 S 3,514,984 6/1970 Cook 3,589,160 6/1971 Gruver et 211.... 3,613,418 10/1971 Nara et a1. 3,604,234. 9/1971 Hinrichson 3,364,713 1/1968 Fukuda et a1... 2,851,042 9/1958 Spence 266/6 S FOREIGN PATENTS OR APPLICATIONS 13,321 6/1969 Japan 72/13 L UWP/QESS. COOL A/VT DESCALE P455 TIME [451 Dec. 18,1973

OTHER PUBLICATIONS Youngstowns 84-ln; Hot Strip Mill Runout Table Strip Cooling System," Iron & Steel Engineer, Sept. 1970, pp. 61-67.

Primary Examiner-Charles W. Lanharn Assistant Examiner-E. M. Combs Attorney-Henry C. Westin [57] ABSTRACT The disclosure of the present invention relates to a strip temperature control method and apparatus for a finishing train of a hot strip rolling mill, including strip interstand coolant supply means, and a control for varying the rate of flow thereof, which includes a feedforward control signal wherein the rate of flow is, controlled in certain stands with reference to a computed temperature and a desired temprature of the strip as it leaves the train, and a feed-backward control signal wherein the rate of How is controlled in one or more stands with reference to the actual temperature and a desired temperature as it leaves the train.

2 Claims, 2 Drawing Figures ADAPT/V5 F COOLANT CONTROL FOR HOT STRIP MILL The presentinvention relates to a control system and method for use in operating a hot strip rolling mill, and particularly, in the operation of the finishing train thereof. It also relates to the employment of interstand coolant in the finishing train and, more particularly, to the employment of high pressure coolant means in the earlier stands and low pressure coolant means in the latter stands and, specifically, the last stand.

Present-day hot rolling mills which employ means for controlling. the temperature of the strip during rolling in the finishing train in order to obtain uniform or constant strip delivery temperature usually provides means for adjusting the speed and, more particularly, the rate of acceleration of the finishing train as a function of the temperature taper in the strip being fed to the finishing train from the delay tablearranged between the finishing train and roughing train. The elimination of the temperature taper effect as it prevents the obtaining of a substantial constant delivery temperature, which results from the fact that the back portion of the strip is exposed to greater heat losses than the front portion and which losses decrease in a gradual manner, is extremely important not only from a metallurgical and quality standpoint, but also in obtaining uniform gauge and ease at which gauge control is obtainable.

A general representation of one form of a presentday method and control system of the type noted above for controlling the strip temperature as a function of acceleration of the finishing train is found in US. Pat. No. 3,4l8,834 which issued to J. W. Cook on Dec. 31, 1968. This patent also contains a discussion of some aspects of the technological background and general considerations involved in temperature control in a hot strip mill. I

One of the most severe disadvantages of such present-day method and control systems is found in the fact that the productivity of the mill is limited in view of the fact that the acceleration as well as the maximum rolling speed of the finishing train are directly tied in with the control of the longitudinal temperature profile of the strip. Because of this, in rolling many products, particularly shorter ones, the finishing train never reaches its top speed capabilities, since the acceleration is controlled to adjust for the longitudinal temperature profile of the strip. It is only in rolling extremely long products, and then only where the temperature difference between the front end and back end of the product is extremely great, that the maximum speed of the train is achieved and even then it is only during the rolling of the last portion of the product. In addition to the production limitations of such systems, because they involve a continuous changing of the mill speed, they create difficulty in obtaining uniform gauge of the strip. From a delivery product standpoint it is well recognized that the ideal rolling condition is when the mill is operated at a constant speed and temperature.

It is, therefore, an object of the present invention to provide a control for the finishing train of a hot strip mill wherein the acceleration of the train is not dependent upon the longitudinal temperature profile of the strip being rolled, but instead the temperature profile thereof is taken care of with reference to maintaining the strip at a substantially uniform delivery temperature by providing strip coolant means and control means for applying'the coolant as a function both of computed and actual temperature values compared with an aimed or desired temperature value.

It is a further object of the present invention to provide at least, in part, for the coolant means to be arranged between the stands on a finishing train, in which a high pressure coolant system is [provided for one or more of the earlier stands and a low pressure coolant system is provided for the latter stands, and particularly, immediately before the last stand.

It is another object of the present invention to include as part of the control means, means for producing a first signal representing the entry temperature of the strip, means for receiving said first signal and computing a second signal representing a computed value of the temperature of the strip at a selected point relative to said train, means for comparing said second sig nal with a third signal representing the desired temperature of the strip at said selected point, means for applying coolant to the strip to reduce any difference between said second and third signals to approximately a desired value, means for producing a fourth signal representative of the actual temperature of the strip at said selected point, means for comparing said fourth signal with a representation of said third signal, and means for applying coolant to the strip to reduce any difference between said third and fourth signals to a desired value.

It is another object of the present invention to include in said control means a first and second means for varying the rate of flow of said coolant applying means. V

It is a still further object of the present invention to employ said interstand coolant means for said first rate of flow control means wherein the: flow control is applied starting from the first stand succeedingly and said second rate of flow control means is applied only to said coolant means for the laststand.

Another object of the present invention is to employ said interstand coolant means for said first rate of flow control wherein the flow control is applied starting from the first stand succeedingly and cumulatively, but excluding one or more of the latter stands, and wherein said second rate of flow control is applied successively and cumulatively to said coolant means for the latter stands.

An additional object of the present invention includes means for initiating. operation of the coolant means associated with the'last stand at a first selected rate of flow and after the comparison of the third and fourth signals at a second rate of flow.

These objects as well as other novel features and advantages of the present invention will be better understood when the following description thereof is read along with the accompanying drawings of which:

FIG. 1 is a diagrammatic view of the last roughing mill stand of a roughing mill train, the delay table, and the finishing train of a hot strip mill, including coolant headers for the finishing train, and an illustrative control system for controlling rate of flow of the headers to offset the longitudinal temperature profile in the strip and permit the mill to be operated at an optimum speed, and

FIG. 2 is a second embodiment involving certain changes to the control system of FIG. 1.

With reference to the strip mill components illustrated in FIG. 1, it is to be understood that, except for the arrangement and use of the coolant supply means, they follow well-known construction and operation.

FIG. 1 shows the last stand of a six-stand hot roughing mill train which is identified as R6, and which is separated from the hot finishing mill train by a delay table 11, consisting, according to well-known practice, of a number of driven support rolls which collectively form a sufficient table length to permit the strip to exit from the roughing stand R6 before entering the first stand of the finishing train 10. Such a strip 14, in exaggerated form, is shown on the delay table 11 in FIG. 1.

The first portion of the finishing train 10 is made up of shear followed by two high-pressure descaling units 1 and 2 arranged in parallel spaced-apart fashion relative to the direction of travel of the strip 14, each having a top and bottom coolant supply header and all of which are located ahead of the first finishing stand, identified as F1. The remaining and succeeding tandemly arranged stands of the finishing train are correspondingly identified as F2 F7, respectively.

The customary drives and speed controls for the stands Fl F7 have not been shown; however, in accordance with the discussion heretofore of the background and objects, it is to be understood that the finishing train is to be driven at basically two speed levels: i.e., at a threading speed and at a maximum or optimum rolling speed with an extremely rapid acceleration from the threading speed to the rolling speed. For illustrative purposes, by way of an example, in the rolling of a mild carbon steel having a delay table length of 270 ft. and

i a delay table thickness of 1.26 in. and a delivery thickness of 0.80 in., the strip would be threaded through the mill and to the coiler, not shown, at a speed of approximately 2,000 f.p.m. Once coiling has commenced, the finishing train speed will be raised to about 4,200 f.p.m. at a typical acceleration rate of 500 ft./min./sec.

Referring again to FIG. 1, between the first four stands or between stands F1 F4;additional high pressure descaling units 4, 5 and 6, respectively, are arranged. These also have top and bottom coolant supply headers as in the case of units 1 and 2. In front of stand F l and between the stands F4 and F5, F5 and F6 and F6 and F7, low pressure strip coolant supply units 3, 7, 8 and 9 are provided. These units also have headers for the top and bottom of the strip. The high pressure units can follow well-known construction and practice, say, for example, as illustrated in US. Pat. No. 3,518,736 that issued to T. C. Domeika on July 7, 1970 which usually operate at a pressure in excess of 2,000 PSI in order to remove scale from the surfaces of the strip. The low pressure units 3, 7, 8 and 9 also can follow the teachings of well-known practice in rolling mills which, by way of example, can follow the type illustrated in FIG. 6 of an article entitled DESIGN AND CONTROL FOR ADVANCED RUNOUT TABLE PROCESSING by F. Hollander appearing in IRON AND STEEL EN- GINEER, March 1971, pp. 81 92, which operate at header pressures less than 10 PSI.

Immediately after the last stand F7 of the finishing train 10, there is provided a thickness indicator in the form of a typical X-ray unit 16 arranged between the rolls ofa runout table 17 comprising, according to wellknown practice, a plurality of driven rolls for conveying the strip to one of several provided downcoilers, not shown.

In still referring to the diagrammatic representation of the mill it will be noted that temperature measuring devices, such as, radiation pyrometers are provided to measure the temperature of the strip as it leaves the roughing stand R6, after the shear 15, and after the last finishing stand F7, which are identified in FIG. 1 as T T, and T respectively. In the illustrated control because of limitations of present instrumentation, the actual temperature represented by T, is not employed, instead, as will be noted hereinafter, a computed temperature value is used to represent the strip temperature entering the finishing train at point T and, more particularly, entering the first of the descaling units 1 and 2.

Continuing the description of the prestand and interstand cooling units, the descaling units 1, 2 and 4 6, in addition to serving to descale the surfaces of the strip, are also employed to assist in controlling the temperature of the strip during rolling at least with respect to the units 4 6. The descaling units 1 and 2 operate together through a single control unit 18 having individual hydraulically operated on-off valves 19. The descaling units 4 6, as well as the unit 3, are individually operable through on-off valves 23 receiving through lines 20, except for the unit 3, high pressure coolant from a common main line 24 and operated by hydraulic units 25. Each of the units 4 6 are under controls 26, the purpose of which will be explained later in connection with the description of the control system.

The descaling units 1, 2, 4 6, it should be noted, are sometimes referred to as high pressure coolant units as distinguished from the units 3, 7 9, which are referred to sometimes herein as low pressure coolant units. The units 3, 7 9, as noted in the aforesaid article, comprise high volume top headers that have the advantage of applying large volumes of water to effect a rapid cooling, but equally important is the uniformity of their application of the coolant across the strip. The bottom headers of the low pressure coolant units because of their disposition, in comparison with the top headers, will operate at slightly higher pressures.

The lines 28 associated with each coolant unit 3, 7 9, while running to the lower headers of the units, are to be understood to represent the piping for the entire unit. The same is true for the lines 20 for the high pressure units. The lines 28 of the units 7 and 8 run to onoff valves 30 operated by controls 31, the unit 32 shown associated with the valves representing sumps. The valves 30 are individually controlled by flow control valves 33, receiving water from a common line 36, which also feeds the unit 3. The valves 33 are controlled by operational amplifiers 37 which, in addition to receiving a control signal, receives a flow error signal from the line 36.

The unit 9 is controlled in somewhat a different fashion since its primary function is to serve as a vernier control for the delivered strip temperature. Accordingly, it is provided with a flow control valve 38 operated by a control 39, the line 28 receiving water from the line 36. The valve 38 is, in return, controlled by an operational amplifier 42 which receives a flow error signal from the line 36 as well as a signal from an operational amplifier 43 which receives a preset signal employed to enable the valve 38 to operate near the midpoint of its total range. In addition a feed-forward signal is received from computer to amplifier 43 over line 45. The amplifier 43 also receives a signal over a line 44. Before leaving the description of the coolant units, it should be noted that while in the illustration given four low pressure units have been shown, dependent upon the circumstances, one or more additional low pressure units could be used in addition to or instead of descaling units 5 and 6, preferably always leaving unit 4 as a descaling unit. The description of the finishing train can be completed by noting that as distinguished from the other stands, stand F5 has provided a signaling device in the form of a load cell 29, the purpose of which will be explained later.

Describing now the control system for regulating the selection and rate of flow of the coolant devices 3 9 to yield uniform strip temperature as the strip leaves stand F7, it will be noted that the control system illustrated in FIG. 1 has conveniently been divided into two separate control circuits 47 and 48, the first being sometimes referred to as a feed-forward system and the latter as relating to a feedbackward system. The control circuit 47 includes an analog computer 49 designed to develop a signal representing a computed value for the temperature of the strip as it enters the finishing train 10, and more particularly at point T, of FIG. 1. As noted before, instead of computing this temperature value, the value may, dependent on development of reliable instrumentation, be obtained directly from a pyrometer located at the point T, which would eliminate the need for computer 49. As the legends appearing on FIG. 1 indicate, the computer 49 receives five electrical signals, the legends representing the following:

V the velocity leaving the last rougher R6,

t, the thickness of the strip leaving R6,

T the temperature of the strip leaving R6,

C the specific heat of the strip,

Time elapsed time from the time of T measurement to position at point T Based on these various signals, the computer 49 will calculate the temperature of the strip at point T, which temperature is identified as cTl and which will involve employment of well-known fundamental laws of heat transfer involving conduction, radiation and convection. A discussion of such laws along with the principles of thermal equivalent of rolling energy as particularly applied to the finishing train of a hot strip mill can be found in many sources for example, an article entitled STRIP TEMPERATURE ANALYSIS IN HOT MILLS by P. C. Thompson et al published in IRON AND STEEL ENGINEER, June 1966, pp. 129 143. The computer 49 sends its signal cTl of the computed value of the temperature at T, to a digital memory unit 50, which in addition receives an electrical signal from a timer 5]. The signal of the computer 49 actually will take the form of a series of discrete signals corresponding to calculated discrete temperatures of succeeding portions of the strip as it passes point T As explained previously, the longitudinal temperature profile of the strip will decrease from the front of the strip to its back so that for various portions of the strip, represented by the letters A to E on the memory unit 50, there normally will be a like number of new computed values for cTl. The function of the timer 51 will be to assure the proper coordination of the various portions of the strip A E to the distinct signals cTl.

Associated with the memory unit 50 is a switch unit 53, the operation of which is controlled by an operational amplifier timer unit 54 that receives a pass time of the coolant the feed-forward circuit 47, and also commences operation of the vernier control unit 9. It will be appreciated that the signal employed to trigger the operation of switch 53 could be produced by other well-known means and that such means as well as the load cell 29 could be associated with one of the earlier stands F l F4. The particular cTl signal from the memory unit 50 for particular strip portions A E is received by a second analog computer 60 designed to develop a computed signal of .the temperature of the strip at point T This signal since it is computed is identified as cT2 in counter-distinction to T which represents the actual temperature of the strip as it leaves the finishing train 10. As legended in FIG. 1 the computer 60 re ceives in addition to the signal cTl five other signals, namely:

z, for the thickness of the strip leaving thelast rougher R6, C for the specific'heat of the strip, 1, for the thickness of the strip leaving the stand F7, as measured by theX-ray 16,

V for the velocity of the strip issuing from stand F7, and

AT an adaptive feedback signal from the control circuit 48, the function of which will be explained later.

Similar to the function of the computer 49, the computer 60 computes a series of discrete signals representing temperatures along the strip at point T2, which signals, identified in legend cT2, are sent to an operational amplifier 61 which also receives a signal representing the desired or aim temperature, identified as T AIM, which is compared with the signal cT2 to produce, if any difference exists, an error signal, legend cT ERR. The error signal, inturn, is sent to an amplifier 62 to effect a progressive and cumulative selection of the cooling units 3 8 dependent upon the required rate of flow necessary to reduce the error signal cT ERR to a desired value, such as, zero. The six lines from the amplifier 62 to the controls 26 and amplifiers 37 of the various cooling units 3 -8 have associated therewith legends 1V, 2V, 3V, 4V, 5 8V and 9 12V, respectively. These legends refer to the strength or largeness of the error signal cT ERR., so that if the signal is only one volt, that is, IV, the unit 3 is the only coolant unit put into operation in addition to the units 1 and 2 which are always on. If the error signal is of a strength of 4V, the units 3 6 will all be operated and in case of a still larger error signal 9 12V, all six of the coolant units 3 8 will come into play.'The computer 60 also sends a signal over the line 45 to initiate operation of the coolant unit 9. I

Turning now to the control circuit 48, it is operated to provide a vernier control for the delivered strip temperature and to effect a recalibration of the value cT2 if the last cooling unit 9 cannot correct for any small differences between the actual strip temperature at point T and the desired or aim temperature at this point. Thus, it serves as a feed-back control, whereas the circuit 47 serves as a feed-forward control with respect to point T2. The control circuit 48, as mentioned before, serves primarily to adjust the variable flow of the coolant unit 9 and for which reason there is provided an operational amplifier 65 which receives a signal fromthe pyrometer arranged at point T2 over line 66 representative of the actual temperature of the strip issuing from the finishing train- 10. It also receives a signal over a line 67 representing the desired or aim of the finishing temperature of the strip indicated in FIG. 1, as in the case of the amplifier 61 as T AIM. The amplifier 65 produces a T ERR. signal which is sent to the amplifier 43 and, hence, to the amplifier 42.

The signal, from the amplifier 65, is compared in amplifier 43 with a signal represneting the mid-range point of the total cooling capacity of the unit 9 and the amplifier 42 compares the signal with a flow error signal on line 36. The signal is sent from the amplifier 42 to the control 39 to regulate the valve 38 to increase or decrease its flow rate, depending upon the error signal T ERR. Flow rate of coolant will be varied to reduce the error signal to a desired value, such as, a zero value. This flow rate adjustment will also be used in the operation of the coolant units 3, 7 and 8.

The T ERR. signal from amplifier 65 is also sent to an amplifier 68, designated in FIG. 1 by the legend HI-LO limiter, suggestive of its function to assure that the T signal will be kept within the range of the coolant capacity of the coolant unit 9 so that it will be able to increase or decrease the coolant effect in serving as a vernier control. To accomplish this a signal from the amplifier 68 is sent over line 69 to the computer 60 as a AT signal, which is also legended as an Adaptive Feed-back signal. This signal will service under the above-stated conditions to override the computer signal cT2 and bring about a correction through altered operation of the feed-forward system. Two circumstances that would bring into play this readjustment to prevent the overriding of the range ofa coolant unit 9 is the incorrect selection of the optimum rolling speed or the fact that one or more of the preceding cooling units failed to turn on.

In briefly describing one mode of operating the illustrated form of the invention, let it be assumed that the strip on the delay table 11 measures 1.26 inches in thickness X 54.3 inches in width and 270 feet in length. The temperature at the head end of the strip, let it be assumed is l,86l F and that of the tail end is l,828 F at point T1. The temperature AIM of the strip for point T is 1,600 F where it will have a finished rolled thickness of 0.080 inch, the threading speed will be 2,000

f.p.m. and the optimum rolling speed at 4,200 f.p.m. During the threading speed period, which will be no more than percent of the final length of the strip, the computer 49 will produce a computed temperature signal CT] for the strip at the entry side of the finishing train at a point T, and at a constant velocity V When the strip reaches stand F5, the load cell 29 will initiate operation of the computer 60-which will then compute the temperature cT2 of the strip at the finishing end of the train at point T2, which signal will be compared in the amplifier 61 with the temperature aim signal for the finishing train (T aim), and as explained heretofore, depending upon the degree or amount of the error signal cT, ERR., one or more of the cooling units 3 8 will be operated. The first unit to be operated will be unit 3 and if its cooling effect is not sufficient to bring about the required correction, then unit 4 will be operated, and so on. Should the cooling units 7 and 8 be required, as noted before, they each have a variable rate of adjustment of their rate of fiow and the range of the unit 7 will be exhausted before the unit 8 is brought into play. The computer 60 will also effect operation of the coolant unit 9 which will be set at the midway point of its total range of coolant capacity. As noted, a series of new calculations of the finishing temperature cT2 are successively computed for the strip entering the mill by the computer 60 so that the amplifier 62 will initiate the required cumulative rate of flow dependent upon the longitudinal temperature profile of the strip entering the mill.

While this temperature control feedforward operation is occurring, the amplifier 65 will continually compare the actual temperature of the strip at T with the temperature aim T at this point and should any difference be determined, which variation, if any, should normally be small, the amplifier 65 will set the required rate of flow of the coolant unit 9 to, in turn, bring about a vernier or fine control of the temperature of the strip leaving the finishing train. Once the threading operation is completed, the finishing train will be rapidly accelerated to the desired and preselected optimum rolling speed. As noted before, in order to obtain maximum production, this acceleration can be as rapid as the drives will allow and, preferably, of the order of 500 ft./min./sec. or higher. The interstand coolant control will continue to operate, as explained during the threading operation, except that the succeeding computed values for the signals for the entry temperature of the strip cTl and finishing temperature cT2 will also reflect the changes in velocities V and V during the accelerating period, the selected constant optimum rolling speed, and the deceleration period. In some cases the back end of the strip may require less coolant and on other occasions may require more, the selected top strip operating speed being the important determinant. In the rolling of strip under the above-stated parameters, while for the front end cooling units 1, 2 and 3 and 9 were operated, for the back end all of the remaining units, i.e., 4 8, were employed to give a substantial constant temperature at point T2 and which was less than 25 F above the aim temperature. This difference was then reduced .to the aim temperature by adjustment of the rate of flow of the coolant unit 9.

FIG. 2 illustrates a second embodiment of the present invention. Since it primarily has to do with the bringing under the control of the feed-back control circuit 48, one or more of the cooling units preceding the unit 9, it involves only a slight change to the feed-forward circuit 47, namely, the elimination of the adaptive feedback signal and AT signal along with the I-II-LO limiter amplifier 68 of the feed-backward circuit 48a. In the embodiment of FIG. 2 the computed signal cT2 produced by the computer 60 is not modified by a signal from the amplifier 65 when more coolant is required than the vernier control unit 9 can produce, instead the preceding units, i.e., 8 etc., are selectively called upon to make the necessary correction for the temperature variation as part of the feed-backward control phase. For purposes of illustration only, FIG. 2 indicates only the last four stands F4a and F7a and coolant units 6a and 9a. Also illustrated there is an X-ray unit 16a and a runout table 17a. The temperature T is still sent to an amplifier 65a and compared with a T aim signal.

The amplifier 65a generates, depending upon the magnitude of the difference or error signal, four control signals indicated on lines 44a, 45a, 46a and 47a. These lines take the place of the line 44 and the lines legended 4V, 5 8V and 9 12V of the FIG. 1 embodiment. The signals operate in generally the same manner as the similar signals of the first embodiment, but in this case in a total feed-back procedure. Small errors between the actual and aim temperatures are taken care of by the coolant unit 9a which still functions as a vernier control and which is turned on by the computer 60, as explained before. When the temperature error signal from the amplifier 67a exceeds the vernier capacity of the coolant unit 9a, the second signal from the amplifier brings into operation cooling unit 8a with the amplifier 43a resetting the rate of flow of the cooling unit 9a to keep it operating at the mean position of its rate of flow range in order that it may continually be employed as a vernier control.

The coolant units 6a and 7a, depending on the mag nitude of the error signal computed by the amplifier 65a, are similarly brought into operation. While the feed-back system 48a is being operated, the feedforward system of the control circuit 47 will continue to compute the cT2 signal for the portions of the strip A E and continue to control the selection and rate flow of the coolant units 3 5, as previously explained.

ln accordance with the provisions of the patent statutes, l have explained the principle and operation of my invention and have illustrated and described what l consider to represent the best embodiment thereof.

I claim:

1. A control system for a rolling mill adapted to reduce the thickness of a hot workpiece comprising:

at least four tandemly arranged stands,

first means arranged between a number of earlier stands of said mill for supplying coolant medium to said workpiece while it is being rolled,

a second means arranged between a number of the latter stands of said mill for supplying coolant medium to said workpiece while it is being rolled,

said first means comprising high pressure coolant means arranged between the first and next adjacent stand of said mill and a low pressure coolant means arranged between two succeeding stands and said second means comprising a low pressure coolan means,

means for controlling the rate of flow of said coolant medium,

said control means includes means for computing the temperature of the workpiece at a first point located after the last stand and producing a first signal representative thereof,

means for comparing said first signal with a second signal representative of a desired temperature at said point after the last stand and for applying coolant medium from said first medium supply means to reduce any difference between the first and sec ond signals to approximately a desired value,

means for producing a third signal representative of the actual temperature of the workpiece at said point after said last stand, and

means for comparing said third signal with a representation of said second signal and for applying coolant medium from said second supply means to reduce any difference between said second and third signals to a desired value.

2. A control system for a rolling mill adapted to reduce the thickness of a hot workpiece comprising:

at least four tandemly arranged stands,

first means arranged between a number of earlier stands of said mill for supplying coolant medium to said workpiece while it is being rolled,

a second means arranged between at least three of the latter stands of said mill for supplying coolant medium to said workpiece while it is being rolled,

means for controlling the rate of flow of said coolant medium including means for selectively operating said second coolant medium supply means progressively and cumulatively starting with said coolant supply means located between the last adjacent stands of said mill,

said control means includes means for computing the temperature of the workpiece at a first point located after the last stand and producing a first signal representative thereof,

means for comparing said first :signal with a second signal representative of a desired temperature at said point after the last stand and for applying coolant medium from said firstmedium supply means to reduce any difference between the first and second signals to approximately a desired value,

means for producing a third signal representative of the actual temperature of the workpiece at said point after said last stand, and

means for comparing said third signal with a representation of said second signal and for applying coolant medium from said second supply means to reduce any difference between said second and third signals to a desired value.

* l l i= 

1. A control system for a rolling mill adapted to reduce the thickness of a hot workpiece comprising: at least four tandemly arranged stands, first means arranged between a number of earlier stands of said mill for supplying coolant medium to said workpiece while it is being rolled, a second means arranged between a number of the latter stands of said mill for supplying coolant medium to said workpiece while it is being rolled, said first means comprising high pressure coolant means arranged between the first and next adjacent stand of said mill and a low pressure coolant means arranged between two succeeding stands and said second means comprising a low pressure coolant means, means for controlling the rate of flow of said coolant medium, said control means includes means for computing the temperature of the workpiece at a first point located after the last stand and producing a first signal representative thereof, means for comparing said first signal with a second signal representative of a desired temperature at said point after the last stand and for applying coolant medium from said first medium supply means to reduce any difference between the first and second signals to approximately a desired value, means for producing a third signal representative of the actual temperature of the workpiece at said point after said last stand, and means for comparing said third signal with a representation of said second signal and for applying coolant medium from said second supply means to reduce any difference between said second and third signals to a desired value.
 2. A control system for a rolling mill adapted to reduce the thickness of a hot workpiece comprising: at least four tandemly arranged stands, first means arranged between a number of earlier stands of said mill for supplying coolant medium to said workpiece while it is being rolled, a second means arranged between at least three of the latter stands of said mill for supplying coolant medium to said workpiece while it is being rolled, means for controlling the rate of flow of said coolant medium including means for selectively operating said second coolant medium supply means progressively and cumulatively starting with said coolant supply means located between the last adjacent stands of said mill, said control means includes means for computing the temperature of the workpiece at a first point located after the last stand and producing a first signal representative thereof, means for comparing said first signal with a second signal representative of a desired temperature at said point after the last stand and for applying coolant medium from said first medium supply means to reduce any difference between the first and second signals to approximately a desired value, means for producing a third signal representative of the actual temperature of the workpiece at said point after said last stand, and means for comparing said third signal with a representation of said second signal and for applying coolant medium from said second supply means to reduce any difference between said second and third signals to a desired value. 